Extracorporeal respiratory support is receiving increasing clinical attention as a treatment for potentially reversible pulmonary failure. Although prolonged perfusion has been applied successfully in the alleviation of acute respiratory distress, it remains an experimental technique and may cause serious clinical complications not directly related to patient pulmonary insufficiency. Crucial questions concerning extracorporeal artificial lung system selection, cannulation hemodynamics, heparinization-coagulation, blood trauma, embolization, and vital organ dysfunction await further definition. A study is proposed to evaluate physiological consequences of prolonged extracorporeal circulation (ECC). Assessment of the possible harmful effects of ECC will be performed comparing baseline veno-venous perfusion, minus oxygenator, with veno-venous study sets incorporating a standard bubbler oxygenator, a homogeneous silicone membrane lung, and a microporous polypropylene membrane lung. Calves of approximately 80 kg will undergo unanesthetized ECC for period longer than 96 hours in determining: blood physical and chemical changes; effects upon specific organs with emphasis on pulmonary, cardiac, kidney, and liver function; and documentation of induced pathological lesions with detailed post-mortem examination. Procedures involving oxygenators will include continual evaluation of gas transfer capability and blood flow resistances as an indication of membrane blood film depositions and subsequent deterioration of oxygenator performance. A Doppler microemboli discriminator will quantify extracorporeal emboli. Chemical analysis and scanning electron microscopy of perfusion circuit components including oxygenators will be performed to identify possible toxic eluants and post-bypass blood-material interfaces. Sterile in vitro bovine blood circulation experiments will complement all long-term ECC test phases in studying protein denaturration, clotting factors, and blood formed element trauma and function under conditions uncomplicated by animal hematological production, repair sequestration, and clearance processes.